This invention relates to metal crucibles for containing materials such as molten magnesium, and to forming steps for use in constructing such crucibles.
Magnesium is used in applications where a metal of low weight and moderate strength is required. Commonly, magnesium articles are made by casting molten magnesium into the desired form.
There are metallurgical problems involved in the handling of molten magnesium. Molten magnesium attacks many metal alloys. Many metals (e.g., nickel, copper) are soluable in magnesium and can contaminate magnesium and make it unsuitable for use. Further, molten magnesium is usually handled at temperatures and in environments which are highly corrosive to many metals.
Magnesium crucibles have been fabricated of metal plate of mild carbon steel or cast steel with negligible nickel content. Such metals, in plate form, are readily cold worked at room temperatures to form component parts of the crucible. The component parts of the crucible are then welded together to complete the crucible.
However, a crucible formed of low alloy steels has a very limited lifetime in the highly corrosive environment of a magnesium melting furnace. The exterior of the crucible is subjected to heavy oxidation (corrosion) resulting in gross scaling of the outer surfaces of the crucible. This scaling is aggravated by intense heating and subsequent cooldown which results in cracking of the scale and the adjacent metal. Scaling not only decreases the efficiency of heat input to the crucible but ultimately results in such degradation of the material that it is either no longer useful or requires extensive reconditioning to make it useful. The lifetime of such a crucible is therefore quite short, often no more than several days.
One technique which has been used to try and reduce the effects of scaling has been to apply a layer of chromium cladding on the external surface of a low alloy crucible. The cladding has been primarily used to try to decrease exterior corrosion. However, there are practical and technical difficulties in using this technique. Technically, the chromium cladding, which is brittle, may crack during application or while in service. Practically (economically), the increased lifetime for magnesium melting crucibles with chromium cladding has not been sufficient to offset the greatly increased cost of the cladding material.